1. Field of the Invention
This invention relates to high efficiency plastic and liquid scintillators which emit light when exposed to ionizing radiation. More particularly, this invention relates to material compositions for providing high-intensity, scintillation light output for making scintillators more sensitive to the presence of ionizing radiation.
2. Description of Related Art
Conventional scintillators have been developed for detection of high energy particles and radiation, such as x-rays, .gamma.-rays, neutrons, and the like. A scintillation detecting system is based on the use of a scintillation composition to convert a portion of the energy imparted to the composition by incident ionizing radiation, to visible or ultra-violet scintillation light. Absolute scintillation (or conversion) efficiency of a composition is defined as the ratio of the energy carried by the visible or ultra-violet light, to the energy lost in the composition by the incident ionizing radiation. The light emerging from a scintillator impinges upon some photo-electric device, e.g., a photomultiplier (PM), or charge coupled device (CCD) where it is converted into an electrical pulse. This electrical pulse is then amplified and recorded by a standard electronic data acquisition system. Details of scintillators in general, and plastic and liquid organic scintillators in particular, are described in publications such as the books by J. B. Birks, "The Theory and Practice of Scintillation Counting", Pergamon Press, (1964), and by G. F. Knoll, "Radiation Detection and Measurement", J. Wiley and Sons 1989 particularly Chapter 8. Plastic scintillators may be a solid sheet or plate or may be in the form of an optical fiber or fiber optic plate such as disclosed in European Patent Publication 0 606 732 A1, Jul. 20, 1994.
Conventional plastic scintillators typically are comprised of a polymeric matrix, e.g., poly(vinyltoluene) (PVT), and one or more fluors (fluorescent compounds), e.g., para-terphenyl (PT) and diphenylstilbene (DPS). Such a scintillator material is haze free, optically transparent, solid and stable. Methods of making and using such conventional plastic scintillators are disclosed in Harrah et al., U.S. Pat. No. 4,594,179. Conventional liquid scintillators, typically are comprised of a liquid solvent matrix, e.g., toluene, and one or more fluors as described supra. Methods of making liquid scintillators are disclosed by J. B. Birks, supra, on pages 273-290.
Generally, a high absolute scintillation efficiency of a scintillator composition is desirable to achieve high detection sensitivity of ionizing radiation. Scintillation efficiency is a function of several parameters, including the type of solid or liquid matrix and the type of fluors employed. Typically, light output relative to anthracene is less than 70% for plastic and less than 80% for liquid scintillators and corresponding absolute scintillation efficiencies are less than about 3% and 4% respectively. Since modern scintillator solute fluors typically have fluorescent quantum efficiencies of close to 100%, a substantial increase in plastic composition scintillation efficiency by alternative choice of fluors is unlikely.
Attempts have been made to increase scintillation efficiency of plastic scintillators by using other plastic matrixes such as polyvinyxylene, polyisopropyl styrene and polyvinyl naphthalene, and copolymers of monomers represented in polymers listed above. Such attempts have resulted in increasing the scintillation efficiency by up to about 50%. Such approaches suffer from one or more disadvantages: the monomers or polymers are commercially unavailable or prohibitively expensive, or polymer compositions are brittle and subject to surface crazing or deterioration.
Addition of naphthalene to liquid and solid scintillators has been explored as a way to increase their scintillation efficiency. Furst et al. (Phys. Rev., 97, 583 (1955)) added large quantities (up to about 25% by weight) of naphthalene to a variety of liquid scintillators to improve their efficiency. The naphthalene was regarded as a secondary solvent. Such mixtures were found to be less prone to impurity quenching (light output reduction due to the presence of impurities). Brown, et al. (Nuclear Electronics 1, 15, 1959)) added naphthalene to solid plastic scintillators where polystyrene (PS) and polymethylmethacrylate (PMMA) were used as matrices. Addition of less than about 3% by weight of naphthalene to a PS mixture containing the fluor 2,5-diphenyl oxazole (PPO), did not chance maximum scintillation efficiency of the mixture. When about 10% by weight of naphthalene is added to PMMA, this polymer is transformed from an extremely inefficient matrix to one with about 50% of the scintillation efficiency of PS.
High concentrations (i.e., between about 5 to 15% by weight) of naphthalene nave been found to embrittle plastic scintillators which dramatically limits its use in fiber optic scintillators where high fiber flexibility typically is required. Also, during high temperature processing of polymeric scintillatots containing naphthalene, such as coextrusion into a clad scintillating optical fiber, the high volatility of naphthalene (boiling point is 217.degree. C.) is found to produce micro bubbles in the material which scatter light and reduce detection efficiency for incident penetrating radiation.
Although advances have been made to produce more efficient solid, liquid and fiber scintillators, there still exists a need to produce scintillator with higher absolute scintillation efficiency; a need to produce flexible plastic scintillating fibers free of micro bubbles, cracks or crazing; and a need to produce such scintillators economically.